Nitride semiconductors are used for: for example, blue LEDs, which are used as light sources of illumination, backlights, or the like; multi-color LEDs; LDs; and the like. Bulk single crystals for the nitride semiconductors, however, are difficult to be manufactured. Instead, GaN is grown on a substrate for growth, such as a sapphire substrate and a SiC substrate, by the MOCVD (metal organic chemical vapor deposition) method.
Above all, sapphire substrates, which exhibit an excellent stability in the high-temperature ammonia atmosphere of the epitaxial growth process, are particularly used as substrates for growth. The sapphire substrates, however, are insulating substrates, and are not electrically conductive. Accordingly, forming two electrodes to sandwich a sapphire substrate is impossible. For this reason, to form a nitride semiconductor on a sapphire substrate, the epitaxial growth is followed, in general, by the etching of the substrate so as to expose the n type gallium nitride layer and by the formation of an n type contact on the etched surface. Thus formed is a structure in which two electrodes p type and of n type are on the same side of a surface.
However, in the configuration described above, that is, the configuration in which two electrodes of p type and n type are formed on the same surface, electric currents tend to concentrate on a mesa portion that is close to the n electrode. The concentration prevents the nitride semiconductor formed on a sapphire substrate from having a higher ESD (electrostatic discharge) voltage. In addition, uniform current injection into the active layer is difficult. This difficulty in turn brings about another difficulty in making the active layer emit light evenly. Moreover, the nitride semiconductor formed on a sapphire substrate needs wire-bonding electrodes, on the same surface, both for the p electrode and for the n electrode. On the other hand, in a nitride semiconductor formed on a conductive substrate, only a wire-bonding electrode for either one of the p electrode and the n electrode has to be formed on a single surface. Accordingly, the nitride semiconductor formed on a sapphire substrate has a narrower effective light-emitting area than the nitride semiconductor formed on a conductive substrate does.
A method used to solve the above-described problem involves removing the sapphire substrate so as to expose the n type gallium nitride layer, and forming an n electrode thereon. For example, there is a method known as the laser lift off (Laser Lift Off: hereafter, abbreviated as LLO) method. In this method, firstly, a compound crystal layer as a nitride semiconductor is formed on a GaN buffer layer that is formed on a sapphire substrate. Then, an excimer laser of, in general, not longer than 300 nm in wavelength, approximately, is irradiated at several hundreds mJ/cm2 from the sapphire-substrate side. The GaN buffer layer is thus decomposed, and then the sapphire substrate is removed (see, for example, Patent Document 1). The chip formed in this method is identical to the chip formed using a GaN substrate. Accordingly, the electrodes can be formed on the opposite sides of the substrate.    Patent Document 1: JP-A-2003-168820    Patent Document 2: JP-A-2004-153271